Electrochemical reactions are among the mechanisms that have been used for selectively displaying visual information ranging from pure color to text or graphics. The visual information can be hidden from sight and later revealed by the electrochemical reactions, or the electrochemical reactions can be controlled to produce the visual information.
For example, U.S. Pat. No. 5,500,759 to Coleman discloses an electrochemical display that can be activated to change color in a predetermined pattern laid out by electrodes. Coleman's design requires an external power source and special electrochromic materials that can be quite expensive. U.S. Pat. No. 4,804,275 to Kang et al. discloses a self-powered electrochromic timing device in which a color change boundary in an electrochromic material is advanced by a gradual dissolution of an electrode. Kang et al.'s electrochromic reactions require a strong acid and other materials that add cost and pose problems for manufacture and use.
Electrode dissolution has also been used in electrochemical display cells to reveal information otherwise obscured by an electrode. For example, U.S. Pat. No. 4,153,345 to Duchene et al. discloses an electrolytic display cell in which a pattern of thin metal film is alternately dissolved into a liquid electrolyte and redeposited onto conductive portions of a transparent electrode. An insulator covers selected portions of the transparent electrode to pattern the depositing metal film. The polarity of an external power source controls whether the display undergoes dissolution or deposition.
Duchene et al. capture the electrolyte between two electrodes. One of the electrodes is a transparent film such as In.sub.2 O.sub.3 or SnO.sub.2 ; and the other electrode, which undergoes dissolution and deposition, is made from a material such as silver. Both electrode materials can be quite expensive, and a separate power source is required to operate the display. Manufacture is also complicated by locating a liquid electrolyte between the two electrodes, requiring each to be supported on a separate substrate.
U.S. Pat. No. 5,339,024 to Kuo et al. discloses a charge indicator cell connected in parallel with a main cell. An anode carried on a conductive substrate of the indicator cell is gradually oxidized to reveal a message written in ink on an underlying layer. One embodiment arranges the electrolyte between the anode and a cathode in the form of a stack. Another embodiment arranges the anode and cathode side-by-side and fashions the electrolyte as a porous film straddling both electrodes. Both embodiments are subject to "islanding", where portions of the anode become electronically isolated from the cathode in advance of the anode's total disappearance. In the side-by-side arrangement, the path of ion conduction becomes increasingly inefficient as the anode erodes further from the cathode. Separate substrates are also used in both embodiments for supporting the electrodes, which complicates manufacture.